1. Field of the Invention
This invention relates to heating systems including an automatically controlled stack damper apparatus, and, more particularly, to a control circuit which provides fail-safe operation of the stack damper apparatus and the fuel ignition and supply apparatus for such systems.
2. Description of the Prior Art
Heating systems employing furnaces having fuel-fired burners require a vent stack to conduct combustion products away from the burner. Many such systems, include an automatically controlled stack damper which permits the vent stack to be closed to minimize heat losses when the furnace is not operating, and to open the vent at the start of each heating cycle. To insure that the stack damper is open in advance of each operation of the burner, systems in which automatic dampers are used generally include an interlock arrangement between the damper control mechanism and fuel supply and ignition apparatus of the system which requires that the damper be fully open before the burner operates.
In one known arrangement in which a primary burner control is conditional on and subsequent to the opening of a stack damper, a drive motor is energized in response to a request for heat and drives the damper to an open position. Limit switches complete the burner circuit and deenergize the drive motor. The motor is energized at the end of the heat run to move the damper to the closed position, and a further limit switch deenergizes the motor when the damper reaches the closed position. Movement of the damper away from its fully open position permits a limit switch to operate and interrupt the burner circuit.
Although such systems prevent operation of the fuel supply apparatus unless the vent stack is open, and maintain the system locked out under certain failure conditions, due to the interlock arrangement, the system may also be locked out following a flame out or a momentary power interruption, an undesirable condition.
A further consideration is that in systems which employ proven pilot type fuel supply apparatus, it is desirable that the pilot valve be deenergized if the pilot fuel fails to be ignited within a predetermined time, commonly referred to as a trial for ignition interval. In one known arrangement, the trial for ignition interval is defined by an electronic timer circuit which controls a solid state switch to effect the deenergization of the pilot valve if a pilot flame fails to be sensed before the end of the trial for ignition interval. However, should the solid state switch fail, the pilot valve will remain operated after the trial for ignition interval, defeating the function of the trial for ignition timer.